1. Plenum Cleaning is Crucial: You stated your mechanic *didn't* clean the plenum. This is a HIGH priority. Carbon buildup in the plenum can severely restrict EGR flow, even with a new valve. The plenum is the intake manifold's upper chamber, and EGR gasses flow through it. A clogged plenum will prevent the system from working correctly, triggering the CEL. This should be done thoroughly.
2. EGR Vacuum System Check: The EGR valve is controlled by vacuum. You need to check:
* Vacuum Lines: Inspect all vacuum lines connected to the EGR valve for cracks, leaks, or disconnections. Replace any damaged lines. A small leak can significantly impact EGR function.
* Vacuum Source: Verify that the vacuum source (usually from the intake manifold) is providing sufficient vacuum. A vacuum gauge can help determine this.
* Vacuum Regulator (if equipped): Some vehicles have a vacuum regulator that controls EGR flow. A faulty regulator can prevent the valve from opening correctly.
3. EGR Position Sensor (if equipped): Some OBD-I systems (like your '94 Ciera) might have an EGR position sensor. A faulty sensor will provide incorrect feedback to the computer, triggering the CEL. This needs to be checked for proper operation and possible replacement.
4. DPFE Sensor (Differential Pressure Feedback EGR Sensor): Many vehicles use a DPFE sensor to monitor EGR flow. A faulty sensor will give incorrect readings, causing the CEL to illuminate. This sensor measures the pressure difference across the EGR valve. This should be inspected and tested.
5. Check Engine Light Code Retrieval: The most important step is obtaining the specific diagnostic trouble code (DTC) that is causing the CEL. While OBD-I systems are less user-friendly than OBD-II, a code reader (or even a paperclip test, depending on the car's OBD-I implementation) can give you a far more precise indication of the problem. This will greatly narrow down the possibilities.
6. Intake Manifold Gasket: A leak in the intake manifold gasket can affect vacuum and EGR operation. While less likely to be the sole cause of the issue, it's worth considering if other checks fail.
7. Computer/ECM Issues (Least Likely): Although less probable given the other possibilities, a faulty engine control module (ECM) could be the culprit. This is usually the last thing to check.
In short: Prioritize plenum cleaning and then thoroughly check the vacuum system and the DPFE sensor (if applicable). Getting the diagnostic trouble code is paramount. With that code, you'll have a much clearer picture of what needs to be addressed. Your mechanic should be able to perform these checks effectively.
